Relationships within the family have been equally controversial. There are many macromorphological characters that unite genera or small groups of genera, such as pseudosyncarpous fruits made up of numerous postgenitally fused carpels in Annona and related ge nera, and a sympetalous corolla and a single pistil with parietal placentation
in Monodora and lsolona, but few that
delimit convincing larger groups. Fries (1959) recognized two major tribes dis tinguished by imbricate or valvate pe tals, hut he immediately had to point out exceptions within each group.
Studies of pollen morphology by Wal ker (197la) led to the first major break though on these questions. Concentra ting on general pollen type and sculp ture as seen under light microscopy and SEM, Walker proposed several trends beginning with monosulcate pollen, as seen in his Malmea tribe: to echinate inaperturate, through irtaperturate to disulculate in his Guatteria tribe, and most strikingly two major trends to te trads : one with coarsely reticulate, co lumellar exine structure, in the Anno- na subfamily, culminating in gigantic polyads in the Cymbopetalum tribe ; the other with no visible exine structure under LM, in the Fusaea subfamily. He thought that the original exine struc ture was columellar and finely reticu late, as in the Malmea tribe, and that the forms with no visible structure, which he called microtectate, were re duced. He believed that the furrow in most of the monosulcate grains was
proximal, rather than distal as in other taxa, based on comparison with the proximal thinning in the tctrad groups. An exception was Pseudoxandra, which has tetrads that show a distal aperture position, and which he therefore pla ced at the base of the family.